IP Multimedia services provide a dynamic combination of voice, video, messaging, data, etc. within the same session. By growing the numbers of basic applications and the media that it is possible to combine, the number of services offered to the end users will grow exponentially, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalized, richer multimedia communication services.
IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over 3G mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) and Service Delivery Protocol (SDP) to set up and control calls or sessions between user terminals (or user terminals and web servers). FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture.
Existing cellular telephone network operators have recently experienced tremendous growth in the numbers of subscribers choosing to use so-called “prepaid” subscriptions; that is where subscribers deposit an amount of cash (the credit balance) with their operators, which is consumed by the subsequent use of services by the subscribers. It is anticipated that the prepaid subscription option will prove equally popular with the users of IPMM services. Indeed, the provision of pre-paid services is likely to be a must for widespread take-up of IPMM services.
When online/real-time charging mechanisms are used (as for prepaid users), the general rule would be for the IPMM Serving Element (SE) providing access to the requested service, to request credit authorisation before granting a mobile node access to the requested service. However, this would inevitably increase the session set-up time for prepaid subscribers, as the IPMM SE must conduct a credit authorization transaction with a Charging Control Node, also referred to as a Prepaid System (PPS) or Online Charging System (OCS).
For some IPMM/IMS based services the session set-up time is critical. This applies for example to so-called Push-to-talk over Cellular (PoC) services such as Instant Personal Talk and Ad-hoc Instant Group Talk, where the originating party pushes the PoC button on his/her terminal to invite one or more users to a walkie-talkie type session, and expects to be immediately in contact with the invited party/parties (as opposed to the traditional call, ring, answer based telephony services). The introduction of the prepaid payment mechanism as currently proposed is likely to result in a degradation of the PoC session set-up time to an unacceptable level, or may lead to a sub-optimal user experience due to the additional delay produced by the credit authorisation phase.